1. Field of the Invention
The present invention relates to a control device and a control method for an automatic transmission that determine whether the vehicle is rapidly decelerating to avoid unnecessary disengagement of a lock-up clutch.
2. Description of the Related Art
Conventionally, a rapid deceleration detection device detects rapid deceleration of a vehicle, a pulse signal in sync with rotation of an output shaft of a transmission is input at first, and the lapse time from the reception of a preceding pulse signal to the reception of a next pulse signal is measured, and the vehicle speed of a predetermined time ago and the time are used to detect rapid deceleration. In addition, a pulse signal in sync with rotation of the output shaft of the transmission is input, the lapse time from the reception of a preceding pulse signal to the present, where a pulse signal is not yet received, is measured, and reduction in the vehicle speed is forecast to detect rapid deceleration even before the reception of a next pulse signal. When rapid deceleration is detected in either of these two ways, the rapid deceleration detection device determines that the vehicle is rapidly decelerating (see Japanese Patent Application Publication No. 2004-116563 (JP-A-2004-116563), for example).
This eliminates the need to wait for the input of a next pulse signal in order to determine the vehicle speed, and allows quick determination of rapid deceleration of the vehicle when waiting a pulse signal at that moment when the vehicle is traveling at a low speed where the frequency of pulses from a rotation sensor is low, thus allowing a lock-up clutch to be disengaged in order to prevent engine stall.
In the rapid deceleration detection device for a vehicle of the above related art, however, rapid deceleration is determined mechanically according to variations in the rotational speed of the output rotary shaft to disengage the lock-up clutch. Therefore, rapid deceleration may be determined in the event of temporary variations in the output rotational speed to unintentionally disengage the lock-up clutch. During highly-responsive gear change control during which determinations for control are made in short periods, for example, rapid deceleration may be determined according to temporary variations in the output rotational speed due to sudden variations in the engine speed, which unintentionally disengages the lock-up clutch.